Strange error in PLSQL script - oracle

Question

I have a strange error on this pl/sql script
'PL/SQL: numeric or value error: character to number conversion error'
in the is_prime function
if((nr mod it) = 0)
HERE->then
return cast(0 as int);
end if;
I can't figure out what I did wrong, because I pass only int's to is_prime and I have declared only a int in is_prime...
set serveroutput on;
drop table exresult;
create table exresult(AA int,BB int);
create or replace function is_prime(nr in int) return int is
it int := 0;
begin
for it in 2..floor(sqrt(nr)) loop
if((nr mod it) = 0)
then
return cast(0 as int);
end if;
end loop;
return cast(1 as int);
end is_prime;
/
create or replace function sum_digits(nr in int) return int is
summ int := 0;
tmp int := nr;
begin
loop
summ := summ + (tmp mod 10);
tmp := floor(tmp / 10);
exit when tmp = 0;
end loop;
return summ;
end sum_digits;
/
declare
target constant int := 5;
nmod int := 0;
nprm int := 0;
it int := 0;
begin
for it in 1..10000 loop
nmod := sum_digits(it) mod 10;
nprm := is_prime(it);
dbms_output.put_line(it + ',' + nmod + ',' + nprm);
if(nmod = target)
then
insert into exresult select it, nprm from dual;
end if;
end loop;
end;

Answer 1

Your problem is actually here:
dbms_output.put_line(it + ',' + nmod + ',' + nprm);
+ is not the concatenation operator in PL/SQL, you need ||:
dbms_output.put_line(it || ',' || nmod || ',' || nprm);
That's relying on implicit conversion of the int values to varchar2, which is fine, but more correctly should be:
dbms_output.put_line(to_char(it) || ',' || to_char(nmod) || ',' || to_char(nprm));